Q12337: Explanation of String Space Garbage Collection; FRE Function

Article: Q12337
Product(s): See article
Version(s): 1.00 1.01 1.02 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 29-DEC-1989

This article discusses three topics regarding string space
compression:

1. When QuickBASIC compresses string space during execution of a
   program

2. Whether disabling string space compression is possible

3. Whether string descriptors are fixed in memory or if they move

This information applies to Microsoft QuickBASIC 4.00 4.00B and 4.50,
to Microsoft BASIC Compiler 6.00 and 6.00B, and to Microsoft BASIC PDS
Version 7.00 for MS-DOS and MS OS/2.

1. When a string variable is assigned a new value, the new string
   takes the next available memory in string space. The old string
   value is deallocated but takes up space until string space "garbage
   collection" occurs.

   A program compresses string space as part of "garbage collection"
   when deallocated strings fill up the 60,000 bytes of available data
   space and the program needs to reallocate a string variable or a
   temporary string variable. You can force string compression with
   the FRE function [PRINT FRE("")], but there is no other way to
   predict when it will occur. An OPEN or CLOSE of a file can also
   trigger garbage collection.

2. There is no way to disable the automatic garbage collection. At
   the time garbage collection occurs, your program is out of memory,
   so that if you were to disable it, your program could not continue
   to execute.

   You can schedule garbage collection to some extent by using the FRE
   function before executing code where you do not want garbage
   collection to occur, but if memory is tight, garbage collection may
   occur again anyway.

3. String descriptors are the first bytes of each string, and they
   move as the strings move.

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